- Aaron Stanford as James Cole
- Amanda Schull as Cassandra Railly
- Todd Stashwick as Theodore Deacon
- Emily Hampshire as Jennifer Goines
- Barbara Sukowa as Katarina Jones
- Tom Noonan as Pallid Man
- Demore Barnes as Marcus Whitley
- Alisen Down as Olivia Kirschner
- Jay Karnes as Robert Gale
- Scottie Thompson as Mantis
- Brooke Williams as Hannah Jones
- Andrew Gillies as Julian Adler
- Murray Furrow as Lasky
- Faran Tahir as Mallick
- Kirk Acevedo as José Ramse
- Abigail Hardinghm as Emma
- Julian Richings as Witness Adviser
Special guest cast
The first scripts for Season 4 were written in March 2017. Filming began in April 2017. The final draft of the script for the series finale, entitled "The Beginning", was completed on June 20, 2017.
|Image||Title||Original Airdate||Episode #||Overall #|
|"The End"||June 15, 2018||1||37|
|"Ouroboros"||June 15, 2018||2||38|
Travelers come face to face with ghosts from their past when they find themselves maneuvering around their own timelines.
|"45 RPM"||June 15, 2018||3||39|
|"Legacy"||June 22, 2018||4||40|
Team Splinter go back in time to the wild frontier in search of a weapon to destroy the Witness.
|"After"||June 22, 2018||5||41|
|"Die Glocke"||June 22, 2018||6||42|
The team infiltrates a Nazi gala to steal an artifact as "Die Glocke"; their operation comes undone when French Resistance interferes and an unexpected guest arrives.
|"Daughters"||June 29, 2018||7||43|
|"Demons"||June 29, 2018||8||44|
Everything changes when the team finally climbs the steps and rings the bell in the Middle Ages.
|"One Minute More"||June 29, 2018||9||45|
|"The Beginning Part 1"||July 6, 2018||10||46|
|The final battle against the Witness as the fate of time and mankind hangs in the balance. All the pieces finally come together to mark the end of the beginning.|
|"The Beginning Part 2"||July 6, 2018||11||47|
When all hope is lost, Cole must complete his cycle so that his younger self will one day arrive at the end with the answer.